Switching power supply for the operation of electric lamps

ABSTRACT

The invention relates to a radio interference suppression circuit for switching power supplies which are suitable for operating both discharge lamps and incandescent lamps. The radio interference suppression circuit has at least one controlled additional source (ZW, F) and one open-loop control circuit (TR1). The additional source or additional sources (ZW, F) generate targeted compensation signals, which are inverted with respect to the interference signals caused by the fast switching transistors (T1, T2). The additional source is either coupled parallel to the interference source and capacitively to the environment (ZW, F) or electrically connected into the supply leads in series with the interference source. In the first case, the additional source preferably includes a metal face (F), mounted in the vicinity of the switches (T1, T2), the potential changes of which face are inverted relative to those of the interference source--which is essentially the region around the junction point (M) of the switching transistors (T1, T2)--for instance by means of a transformer (TR1). In the second case, the additional sources are preferably formed by two secondary windings of the transformer. The primary winding is triggered by the potential changes of the interference source and in the secondary--side supply leads induces a phase-opposition common mode compensation signal. Optionally, the interference suppression can also be in the form of a closed-loop control circuit, in which the common-mode noise of the supply leads acts as the controlled variable.

FIELD OF THE INVENTION

The invention relates to switching power supplies for operating electriclamps connected to an alternating voltage mains network or a directvoltage source, and more particularly to a radio noise suppressioncircuit thereof.

BACKGROUND

Switching power supplies are suitable both for operating dischargelamps, especially fluorescent lamps and high-pressure lamps, and foroperating incandescent lamps, such as low-voltage incandescent halogenlamps. Switching power supplies for operating discharge lamps aregenerally called electronic ballast devices (EBDs), while forlow-voltage incandescent halogen lamps, the term electronic transformeror electronic converter is conventional.

Switching power supplies supplied by an alternating voltage mainsnetwork are also known as on-line switched-mode power supplies. If aswitched-mode power supply is connected to a direct voltage source, suchas a battery, then it can also be called an off-line switch-mode powersupply. It is also possible to connect two or more switching powersupplies in the manner of a cascade circuit; the output of a precedingpower supply is then connected to the input of the next power supply,and so forth. In European Patent Disclosure EP-A 0 485 865, forinstance, a circuit arrangement for operating a discharge lamp is shown.The circuit arrangement is supplied from a direct voltage source, suchas an on-board electrical system of a motor vehicle, and has a step-upconverter (upward controller) and optionally a downstream inverter foroperating a discharge lamp with alternating current (AC).

One essential feature of switching power supplies is at least oneswitching portion having one or more fast switches--for which fastswitching transistors are used as a rule. The switching portion may--forinstance as explained in W. Hirschmann and A. Hauenstein,Schaltnetzteile Switched-Mode Power Supplies!, Siemens AG, Berlin, 1990,page 40 ff.--be in the form of a choke converter (downward, upward, orupward-downward controller, inverter), flyback converter, forwardconverter, or push-pull converter (in half- and full-bridge circuit).The switching portion converts the voltage at its input, such as therectified mains voltage or the output voltage of a preceding converter,into a high-frequency switched voltage. Especially in the region of theswitches, this creates fast potential changes relative to the groundedhousing mass (protection class I devices) or the environment or ground(protection class II devices). Via capacitive couplings, the electricalfields, which vary over time, connected to the potential changes canaffect common-mode noise, or interference, which for instance flows overthe mains supply leads and through the switching power supply. The noiseloop is closed via parasitic capacitances especially between theswitching portion and ground. A detailed description of how radio noisearises can be found for instance in W. Hirschmann and A. Hauenstein,Schaltnetzteile Switched-Mode Power Supplies!, Siemens AG, Berlin, 1990,page 72 ff. With respect to the limit values for radio noise inswitching power supplies, VDE Specification 0871, and especially forelectrical operating devices for lamps, VDE 0875--which corresponds toInternational Standard CISPR 15--must be adhered to.

One conventional provision to suppress common-mode noise is toincorporate an interference suppression filter, such as acurrent-compensated choke, into the mains supply leads. The design ofcurrent-compensated chokes is explained for instance in O. Kilgenstein,Schaltnetzteiie in der Praxis Switched-Mode Power Supplies in Practice!,Vogel Buchverlag, Wurzburg, 1986, p. 355 ff. Its effect is based on thefact that the mains-frequency useful current can pass through undamped.High-frequency common-mode noise, conversely, is filtered out by thehigh inductance of the current-compensated choke. However, there arelimits to compact structure, since the interference-suppressing actionof a current-compensated choke can be reduced by immediately adjacentcomponents and their noise signals or even--especially because ofmagnetic interference fields--be converted into an opposite kind ofaction.

In protection class I devices, Y capacitors can additionally beconnected from the mains supply leads to the protective or groundconductors; as a result, at least some of the common-mode noise can flowaway to ground. This possibility does not exist with protection class IIdevices.

European Patent EP 0 264 765 to which U.S. Pat. No. 4,862,041corresponds, describes an electronic converter for operating low-voltageincandescent halogen lamps, which has a current-compensated choke forsuppressing radio interference. The secondary side of the powertransformer--which acts as a decoupling circuit that transforms theswitched voltage of the switching portion to the rated voltage of thelow-voltage incandescent halogen lamps connected to it--is alsoconnected via a capacitor to the positive or negative pole of the mainsrectifier. As a result, an HF short circuit is created. which keepsinterference voltage across the power transformer low. However, thisprovision is limited to electronic converters.

German Patent Disclosure DE-OS 41 37 207 discloses an HF interferencesuppressor that is also based on an HF short circuit and that can inprinciple be used both in EBDs and in electronic converters. To thatend, an HF signal, in the case of an EBD, is for instance decoupled fromthe series resonant circuit of the discharge lamp and connected via ahigh-pass filter to an interference-suppression choke connected to themains supply leads. If the high-pass filter is optimally dimensioned,virtually no noise currents flow via the mains supply leads. However,the hf impedance of the interference suppression choke varies as afunction of the value of the input current flowing through it. As aresult, the interference suppression action varies sensitively with theload connected.

THE INVENTION

The object of the invention is to overcome these disadvantages and todisclose switching power supplies for operating electric lamps whosecommon-mode radio line noise is below the limit values for pertinentspecifications. Moreover, the radio interference suppression circuitshould enable a compact design of the power supply and should besuitable in principle for both electronic converters and electronicballast devices. Another aspect of the object is to disclose anespecially economical version with as few additional components aspossible.

Briefly, the fundamental concept of the present invention is tocompensate for interference signals--interference voltages andcommon-mode noise--by means of one or more suitable, controllableadditional source or additional sources. Compensation signals that areinverted relative to the interference signals are generated by theadditional source or additional sources. By superposition, the twosignals ideally cancel one another out entirely. The effectiveness ofthe compensation is variable by means of the amplitude, frequency andcourse over time of the compensation signal and its relative phaserelationship to the interference signal. In an open-loop controlledversion of switching power supplies of the invention, these influencingvariables are fixedly set. In a closed-loop controlled version, thecontrolled variable is suitably obtained from the common-mode noise thatflows in the mains supply leads or in the input-side connecting leads ofthe switching portion. Preferably, the closed-loop control is set suchthat the aforementioned interfering signals are minimized.

The controllable additional source--which functions essentially as avoltage source--is connected either parallel or serially to theinterference source, or in other words to the switching portion. In thefirst case, an additional source in the form of a voltage generator anda coupling element connected to it is disposed in the vicinity of theswitches, and thus similarly to the interference source is coupledcapacitively to the environment, such as the ground. The interferencecurrent flowing from the interference source to the environment viaparasitic capacitances is compensated for in that an invertedcompensation current--also flowing to the environment via theseparasitic capacitances, is influenced by the additional source. This isaccomplished by targeted potential changes of the additional source,which are inverted relative to those of the interference source. Thismeans that the courses over time of the two potential changes areidentical except for a phase rotation of typically 180°.

In the second case, the supply leads of the switching portion, comingfrom an alternating voltage mains network or a direct voltage source,each have one additional source. If the switching power supply issupplied from a alternating voltage mains, then the additional sourcescan selectively be connected serially into the input or output leads ofa mains rectifier. By suitable triggering, each of these additionalsources then generates a compensation voltage, which is invertedrelative to the interference voltage generated by the interferencesource. The compensation voltage and the interference voltage areadapted to one another such that ideally they compensate for one anothercompletely, and consequently the creation of common-mode noise isaverted. The phase-locked coupling between the compensation signal andthe interfering signal is assured in both cases by means of asynchronizing signal obtained from the interference source, whichsignal, in the case where a push-pull converter is used, is picked upfor instance at the center point between two bridge transistors.

The coupling element of the parallel additional source is formed by anequipotential face, that is, an electrically conductive surface, forinstance of metal or conductive plastic. It is connected to one pole ofthe controllable voltage generator and acts like one half of a platecapacitor coupled parasitically to the environment. The equipotentialface is advantageously located in the vicinity of the switching portion,preferably on the printed circuit board of the switching power supply.This assures that a change in parasitic capacitive couplings--forinstance by placing the entire circuit arrangement in a grounded metalhousing--occurs to the same extent for both the interference source andthe equipotential face of the additional source, and consequently thecompensation is preserved approximately without change. The potentialchanges of the equipotential face required to generate a compensationcurrent are brought about by the voltage generator, suitably triggeredwith the aid of the open-loop control circuit; the level of theinfluenced compensation current can be varied, among other provisions,by means of the geometrical dimensions of the equipotential face andespecially its surface area.

In a first embodiment, the open-loop control circuit comprises aninverter stage and a downstream driver stage. For synchronization, theinput of the inverter stage is connected to the switching portion. Theinverted synchronization signal is supplied via the driver stage to thedownstream controllable voltage generator, which comprises a push-pullend stage. The driver stage--preferably in the form of an operationalamplifier--controls the complementary transistors of the push-pull endstage. The output of the push-pull end stage is connected to anequipotential face and changes its potential inversely to thesynchronization signal. The voltage supply for the push-pull end stageis preferably drawn from the voltage supply of the switching portion, inthis way, potential changes can be generated on the equipotential facewhose amplitudes correspond to those of the interference source. By asuitable choice of the transistors of the push-pull end stage and theirtriggering, it is assured that the rises in the potential changescorrespond to the steep switching edges of the transistors of theswitching portion. In a variant, the complementary transistors arereplaced by identical transistors. However, this would requireseparate-potential trigger signals for the transistors, which means morecomponents and consequently higher costs.

In a second embodiment, the open-loop control circuit comprises theprimary winding of a transformer, and the voltage generator comprisesthe secondary winding of the transformer. For synchronization with theinterference signal, the primary winding is connected to the switchingportion. The secondary winding is connected on its first end to anequipotential face and on its second end, which is the base point, to asuitable potential of the circuit in such a way that together with adirection of the winding of the transformer, an inversion between thesynchronizing signal and the voltage signal of the secondary side isattained. The compensation current can be varied by means of the step-upratio of the transformer and the size of the equipotential face.

Typically, switching power supplies contain inductive components thatare connected to the switching portion, examples being lamp chokes inEBDs, converter chokes in blocking converters, or power transformers inelectronic converters. In a preferred embodiment, these inductivecomponents are simultaneously jointly used as an open-loop controlcircuit for the controllable additional source, or in other words as acomponent of the aforementioned transformer. In an especially preferredvariant for electronic converters, the voltage generator is formed as acontrary--direction additional winding on the core of the transformer,the first end of which winding is connected to the equipotential face.This represents an especially inexpensive, compact version of the radiointerference suppression circuit, since only one additional winding butno additional components are needed. The equipotential face can beattained and again way and again without additional expense in the formof a metal face by means of a suitable layout on the copper-linedprinted circuit board of the switching power supply. The base points ofthe additional winding and of the primary winding of the transformer arepreferably connected to one another. The metal face is connected to thefree end of the additional winding. The compensation can be varied notonly by the size of the metal face but also by the number of windings ofthe additional winding and by the choice of potential of the base pointof the additional winding.

In a preferred embodiment of the second case, the serial additionalsources are formed by the secondary windings of atransformer--hereinafter called a feed transformer; one secondarywinding is connected serially into each supply lead of the switchingportion coming from a voltage source. The number of secondary windingsis accordingly equal to the number of supply leads. Optionally, a mainsrectifier precedes or follows the additional sources as well. Theopen-loop control circuit comprises the primary winding of the feedtransformer, which is coupled with the secondary windings in theopposite direction. The primary winding is connected to the switchingportion, for instance to a junction point of the switches--optionallyvia an additional impedance. In this way, the synchronizing signals thatflow through the primary winding induce compensation voltages in thesecondary windings; these compensation voltages are inverted relative tothe interference voltages of the switching portion. The compensation canbe varied by the dimensioning of the feed transformer (step-up ratio,direction of winding) and optionally of the additional impedance.

The closed-loop controlled version of the various embodiments of theradio interference suppression circuit of the invention are attained asexplained below. In the circuit arrangements described above, atransformer--hereinafter called a sensor transformer--is additionallyprovided. Its primary side comprises two windings which are eachserially connected into the mains supply leads or the switching portionsupply leads. The secondary side comprises one winding and is connectedto the inputs of a closed-loop control amplifier. The direction ofwinding is designed such that only common-mode noise on the supply leadsof the primary side induce a significant, phase-opposition signal in thesecondary winding. In the closed-loop controlled version, the inverterstage, which is needed in the case of the parallel-connected open-loopcontrolled additional source (push-pull end stage with equipotentialface) can thus be dispensed with. As the closed-loop control amplifier,the driver stage already described in the open-loop controlled versionis used. It controls a downstream push-pull end stage, which in turn isconnected to the equipotential face. Via the capacitive coupling of theequipotential face to the environment, a common-mode compensationcurrent is generated, which closes the control loop. In the case ofserially connected additional sources, the push-pull end stage andequipotential face are replaced by the feed transformer.

A decisive advantage of the invention should be mentioned expresslyagain in this connection. Although both the two primary windings of thesensor transformer and the two secondary windings of the feedtransformer each act as current-compensated chokes, nevertheless aresultant damping of common-mode noise is unnecessary for thecompensatory action of the radio interference suppression circuit. Forthe idling inductances of the windings, values can therefore be chosenthat are far lower (for instance, 1 mH) than those of conventionalcurrent-compensated chokes (such as 20 mH). As a result, decisivelymore-compact dimensions of the circuit arrangement are attained.

The reduction in common-mode noise by the phase-opposed currents(parallel additional source) or voltages (serial additional sources) isapproximately proportional to the circuit gain of the control loop. Thecircuit gain K is composed of the transmission factors of thetransformers connected in the control loop and the gain of theclosed-loop control amplifier. For K>50, a reduction in the common-modenoise that is already adequate for the pertinent regulations isattained. The bandwidth of the circuit gain should be chosen such thatthe expected interference spectrum can be compensated for, or in otherwords cancelled out.

DRAWINGS

The invention will be described in further detail below in terms ofseveral exemplary embodiments. Shown are:

FIG. 1, the circuit diagram of an electronic converter according to theinvention for low-voltage incandescent halogen lamps, with an open-loopcontrolled radio interference suppression circuit which has anadditional winding and a metal face, connected to it, as an additionalsource connected parallel to the interference source;

FIG. 2, the layout of the printed circuit board equipped in accordancewith the electrical circuit of FIG. 1;

FIG. 3a, the interference spectrum, measured by the CISPR 15 Standard,of the electrical circuit of FIG. 2 in the range between 150 kHz and 30MHz, in which the power transformer is connected to a 100 W incandescenthalogen lamp over supply leads approximately 2 m long;

FIG. 3b, the interference spectrum of FIG. 3a, but in the range between50 kHz and 1.4 MHz;

FIG. 3c, the interference spectrum of FIG. 3a, but in the range between20 kHz and 160 kHz;

FIG. 4, the basic circuit diagram of an electronic converter accordingto the invention for low-voltage incandescent halogen lamps, with anopen-loop controlled radio interference suppression circuit, which has afeed transformer as additional sources connected serially to theinterference source;

FIG. 5, the basic circuit diagram of an open-loop controlled radiointerference suppression circuit, which has a push-pull end stage and ametal face connected to it as a parallel-connected additional source;

FIG. 6, the closed-loop controlled variant of the radio interferencesuppression circuit of FIG. 5;

FIG. 7, the closed-loop controlled variant of the radio interferencesuppression circuit of FIG. 4.

DETAILED DESCRIPTION

FIG. 1 shows the circuit diagram of an electronic converter for 12 Vincandescent halogen lamps with a maximum power consumption of 105 W.The individual components will be explained below. The input of thecircuit is protected by the fuse S11 and the thermal switch TS1 againstexcessive currents and excessive temperature. This is followed by adifferential interference suppression filter, comprising the X capacitorC12 and the double choke L1. One of each of the two windings of thedouble choke L1 is connected serially with one each of the two supplyleads of the input and is damped each with a parallel resistor R12, R13.This is followed by a mains rectifier, comprising the diodes D1-D4 in abridge circuit, whose output has a parallel-connected filter capacitorC14 and a varistor VAR as overvoltage protection.

This is followed by a free-running current-feedback inverter in ahalf-bridge circuit. Its function is extensively described in publishedGerman Patent Application DE 44 16 401 A1 by the inventor hereof. ThisPublication relates to a circuit to operate electric lamps which,essentially, includes a rectifier circuit, an inverter connected as ahalf-bridge or a full-bridge, which is self-oscillating and currentfeedback coupled, and a trigger generator, which has at least aresistor, a charge capacitor and a voltage dependent switching element,such a diac. A controlled discharge resistor, particularly a transistor,is coupled in parallel to the charge capacitor. The discharge resistoris controlled by the inverter. When the inverter oscillates, the chargecapacitor is discharged through the discharge resistor and therebyprevents undesired generation of trigger pulses. Therefore only theindividual functional groups and components will be described below. Itskey part is a free-running, current-feedback half-bridge converter,essentially comprising the two switching transistors T1, T2--to each ofwhich one return diode D7, D8 is connected parallel; the two bridgecapacitors C8, C9, each with one parallel-connected diode D9, D10; thepower transformer TR1, whose secondary winding is connected to thepositive pole of the mains rectifier via the radio interferencesuppression capacitors C6, C7 and to which a 12 V incandescent halogenlamp is connected; and a control transmitter STR1-A through STR1-C,which furnishes the control signals for the bridge transistors T1, T2,which signals are supplied, each via a trigger circuit comprising theresistors R2, R4 and R3, RS and the diodes D15 and D16, respectively, tothe base terminals of the bridge transistors T1 and T2. The half-bridgeconverter is started by means of a conventional trigger generator, whichessentially comprises the resistors R1, R7, the charge capacitor C2, andthe diac DIAC1. The transistor T4, connected parallel to the chargecapacitor C2, whose base is connected via the resistor R14 to the baseof the half-bridge transistor T2, prevents the occurrence of undesiredtrigger pulses while the half-bridge converter is oscillating. Thischaracteristic is already disclosed in German Patent Application DE 4416 40A1. A protection circuit known per se, substantially comprising thetransistor T3, the capacitors C3, C5, the diode D11 and the resistorsR6, R8 and R11, protects the bridge transistors T1, T2 againstsecondary-side overload. In accordance with a feature of the invention,the radio interference suppression circuit comprises an additionalwinding ZW on the power transformer TR1 and a metal face F. Theadditional winding ZW and the primary winding of the power transformerTR1 have opposite winding directions and each comprise 73 windings. Theyare connected to one another at the base point FP. The free end of theadditional winding ZW is contacted to the metal face F. In this way, thepotential changes of the center point M between the two bridgetransistors T1, T2 are transmitted in phase opposition to the metal faceF. The components used are listed in Table 1.

FIG. 2 shows the layout of the printed circuit board, produced andequipped in accordance with FIG. 1. In a known manner, radiointerference is dependent to a high degree on parasitic capacitances andinductances and consequently on the components used, theirthree-dimensional arrangement, and their electrical connections to oneanother. The metal face F is therefore carefully adapted to this layout.The production of the metal face F is effected upon etching of theprinted circuit board directly out of the copper coating. Thus there areneither additional production expenses nor additional material costs.This is accordingly an especially economical form of the interferencesuppression circuit according to the invention.

In FIGS. 3a-3b, the quasi-peak (QP) interference spectra of the electriccircuit of FIG. 2 are shown, measured in accordance with thespecifications of CISPR 15. During the measurement, a 100 W incandescenthalogen lamp is operated with supply leads, approximately 2 m long, thatare connected to the additional winding ZW of the power transformer TR1.It can clearly be seen that the measured values over the entiremeasurement range (20 kHz to 30 MHz) are at times considerably below theCISPR 15 limit value line G shown. FIG. 3a shows the course of the QPmeasurement signal as a function of the frequency in the range between150 kHz and 30 MHz. FIGS. 3b and 3c correspondingly show the rangesbetween 50 kHz and 1.4 MHz, and between 20 kHz and 160 kHz,respectively. In the last illustration, the fundamental frequency of theinverter at approximately 32 kHz and the two subsequent harmonics attwice and four times the fundamental frequency, respectively, arereadily visible.

FIG. 4 shows the basic circuit diagram of an electronic converter with acontrolled serial additional source. The circuit comprises the followingcomponents: feed transformer ES, mains rectifier GR, trigger generatorTG, and self-excited current-feedback half-bridge converter HB. The feedtransformer ES comprises a primary winding WE1 and two secondarywindings WE2 and WE3 that are connected serially to the mains supplyleads N. The half-bridge converter HB substantially comprises the twobridge transistors T1 and T2, the two bridge capacitors C2 and C3, thepower transformer TR1, and the control transmitter RK-a through RK-c.The rectifier GR and trigger generator TG correspond to those of FIG. 1and are merely shown as function blocks for the sake of simplicity. Thesecondary windings WE2 and WE3 are connected by their first ends C andD, respectively, to the mains N and by their second ends c and d to theinputs G1 and G2, respectively, of the mains rectifier GR. The primarywinding WE1 is connected by its first end to the input E2 of thehalf-bridge converter HB. The second end is connected via the impedanceZ to the center point M of the two half-bridge transistors T1 and T2. Inthis way, a synchronizing current that is proportional to theinterference potential of the center point M flows in the primarywinding WE1 and in each of the secondary windings WE2 and WE3 induces aphase-opposed compensation voltage. The amplitude of the compensationvoltage is adapted to the interference potential by means of theimpedance Z in such a way that the two voltages compensate for oneanother, thus averting the occurrence of common-mode noise. In avariant, the feed transformer ES is connected not in the mains supplyleads N but rather between the mains rectifier GR and the input of thehalf-bridge converter HB. In that case, the terminals C, D and c, d ofthe two secondary windings of the feed transformer ES are connected tothe two outputs G3, G4 of the mains rectifier GR and the inputs E1 andE2 of the half-bridge converter HB, respectively. The terminals of theprimary winding remain unchanged.

FIG. 5 shows a further exemplary embodiment of an open-loop controlledradio interference suppression circuit, which has a push-pull end stageGE and a metal face F connected to it as a parallel-connected additionalsource. The push-pull end stage substantially comprises the twocomplementary transistors T5 and T6 and one basic connection each bymeans of a resistor R15 and a diode D16. The metal face F is contactedto the center point M2 between the two complementary transistors T5 andT6. The open-loop control circuit comprises the inverter stage I and thedriver stage T and it triggers the push-pull end stage. This radiointerference suppression circuit can be built into the electronicconverter of FIG. 1, for instance. In that case, the additional windingZW can be omitted. The control input S is connected to the center pointM of the bridge transistors, and the + and - terminals of the voltagesupply of the push-pull end stage GE are connected respectively to theoutputs G3 and G4 of the mains rectifier GR.

FIG. 6 shows a further exemplary embodiment of a radio interferencesuppression circuit, which has a push-pull end stage GE and a metal faceF connected to it, as a parallel-connected additional source. UnlikeFIG. 5, here the circuit is designed as a closed-loop control circuit.To that end, the driver stage T is supplied from the secondary windingWS3 of a sensor transformer SE. The primary side of the sensortransformer SE has two windings WS1 and WS2, whose winding direction isadapted to the secondary winding WS3 in such a way that onlyprimary-side common- mode noise generates a secondary-side signal. Tothat end, the first ends A and B of the two primary windings WS1 and WS2are connected to the two mains supply leads N or alternatively to thetwo outputs G3 and G4, respectively, of the mains rectifier GR. The twoother ends a and b of the secondary windings are contacted to the inputsG1, G2 of the mains rectifier GR, or alternatively to the inputs E1, E2of the half-bridge converter HB.

FIG. 7, finally, shows an exemplary embodiment of a closed-loopcontrolled radio interference suppression circuit, in which twosecondary windings of a transformer are used as serially connectedadditional sources. This circuit has both a sensor transformer SE as inFIG. 6 and a feed transformer ES as in FIG. 4. The terminals a and c, onthe one hand, and b and d, on the other, of the windings, seriallyconnected into the supply leads, of the two transformers are connectedto one another. The secondary winding WS3 of the sensor transformer SEis connected via the driver stage T to the primary winding WE1 of thefeed transformer ES. The driver stage is formed by an operationalamplifier, whose inputs are connected to one another via the resistorRT. The circuit may for instance precede or follow the mains rectifierGR. In the first case, in FIG. 4, the interfaces C, D and c, d aredisconnected, and instead of the feed transformer ES, the terminals A, Band C, D of the circuit of FIG. 7 are connected to the mains supplyleads N or to the inputs G1, G2 of the mains rectifier GR. In the secondcase, in FIG. 4 the connections between G3, E1 and G4, E2 aredisconnected, and instead the terminals A, B and C, D of the circuit ofFIG. 7 are connected to the outputs G3, G4 of the mains rectifier GR orto the inputs E1, E2 of the half-bridge converter. The feed transformerES is removed from the circuit, and the interfaces C, c and D, d areconnected to one another. The dimensioning of the closed-loop controlcircuit can be seen from Table 1.

The invention is not limited to the exemplary embodiments described. Inparticular, individual characteristics of different exemplaryembodiments may also be combined with one another. Moreover, theinverters mentioned may also be in the form of a full-bridge circuitand/or the decoupling circuit may be a resonant circuit for igniting andoperating discharge lamps, without in principle requiring any change inthe interference suppression circuit, or any loss of its advantageouseffect. Finally, it is also possible for the exemplary embodiments ofradio interference suppression circuits described to be installed,without fundamental changes, in other embodiments of a switching powersupply, such as in choke converters (downward, upward, upward-downwardcontrollers, inverters), flyback converters, and flow converters. Inaddition, identical or different forms of switching power supplies mayalso be combined with one another in the manner of a cascade circuit,with each individual switching power supply having its own radiointerference suppression circuit.

                  TABLE 1                                                         ______________________________________                                        Component list for the circuit of FIG. 1                                      ______________________________________                                        SI1          T 1A                                                             R1           165 kΩ                                                     R2, R3       3,3 Ω                                                      R4, R5       100 Ω                                                      R6           560 Ω                                                      R7           165 kΩ                                                     R8           51 kΩ                                                      R9           330 kΩ                                                     R11          240 kΩ                                                     R12, R13     10 kΩ                                                      R14          1 kΩ                                                       C2           10 nF; 250 V                                                     C3           47 μF; 6,3 V                                                  C5           6,7 nF; 400 V                                                    C6, C7       470 pF; 2 kV                                                     C8, C9       88 nF; 400 V                                                     C12          220 nF; 250 V                                                    C14          150 nF; 400 V                                                    D1-D4        1N4007                                                           D7-D10       1N4937                                                           D11, D14, D15                                                                              LL4148                                                           DIAC1        NEC, 32 V                                                        L1           BVL32                                                            STR1-A-STR1-C                                                                              EF16 4.5/15.5/15.5 windings                                      TR1          R26 73/73/8 windings                                             T1, T2       SGS F343                                                         T3, T4       BC850C                                                           VAR          S10 K250                                                         ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Component list for the circuit of FIG. 7                                      ______________________________________                                        WS1, WS2      2 windings                                                      WS3           50 windings                                                     RT            47 kΩ                                                     T             AD 844                                                          WE1           5 windings                                                      WE2, WE3      50 windings                                                     ______________________________________                                    

We claim:
 1. A switching power supply for the operation of electriclamps connected to an alternating voltage mains network or directvoltage source, comprisinga switching portion (HB), which contains oneor more switches (T1, T2) for generating a switched voltage, and whichbecause of unavoidable parasitic capacitances also acts as aninterference source capacitively coupled to the environment; a radiointerference suppression circuit for reducing interference signalsgenerated by the switching portion, a trigger (TG; STR1-A-STR1-C;RK-a-RK-c) or controller for the switching portion (HB), a decouplingcircuit (TR1), to which at least one electric lamp (HG) is connectedeither indirectly, via optionally one or more downstream furtherswitching power supply or switching power supplies, or directly,characterized in that the radio interference suppression circuit has oneor more controlled additional source (or additional sources) (ZW, F; GE,F; WE2; WE3) and a control circuit (TR1; WE1; I, T) connected to theswitching portion and synchronizing the additional source or additionalsources, wherein said control circuit comprises one of an open-loopcontrol circuit (TR1, WE1, I, T) and a closed-loop control circuit (SE,T, WE1), wherein the additional source (or additional sources) and thecontrol circuit contain an inverter and generating one compensationsignal per additional source (ZW, F; GE, F; WE2; WE3), whichcompensation signal is inverted relative to the interference signalgenerated by the switching portion; and wherein the additional source(or sources) is (are) connected either parallel or serially to theinterference source, whereby a compensation between the interferencesignal and the compensation signal is attained.
 2. The switching powersupply of claim 1, wherein the control circuit comprises the open-loopcontrol circuit (TR1, WE1, I, T), and said open-loop control circuit isconnected to a tap (M) of one or more switches (T1, T2) of the switchingportion, whereby the open-loop control circuit (TR1; WE1) is suppliedwith a synchronizing signal required for the synchronization.
 3. Theswitching power supply of claim 2, wherein the power supply includes abridge circuit (HB) having transistors, and wherein the junction point(M) of two bridge circuit transistors (T1; T2) forms the tap.
 4. Theswitching power supply of claim 1, wherein the interference source andthe additional source are connected in parallel, andthe additionalsource has a controllable voltage generator (ZW; GE) and anequipotential face (F) connected to the voltage generator, and theequipotential face functions like one-half of a plate capacitorparasitically coupled to the environment.
 5. The switching power supplyof claim 4, wherein the equipotential face is formed by a metal face (F)disposed in the vicinity of the switching portion (HB), and theopen-loop control circuit (TR1; I, T) contains the inverter, and thepotential changes of the metal face are inverted relative to thepotential changes of the tap (M) in the switching portion (HB).
 6. Theswitching power supply of claim 5, wherein the metal face (F) is part ofa printed circuit board on which the switching power supply isassembled, and the surface area and shape are adapted to the layout ofthe printed circuit board and the components used in such a way thatminimization of the interfering signals is attained.
 7. The switchingpower supply of claim 4, wherein the control circuit comprises theopen-loop control circuit and said control circuit has, in addition tothe inverter (I), a driver stage (T), andwherein the voltage generatoris connected to the open-loop control circuit and has an end stage (GE);and wherein the inverter (I) is supplied with the synchronizationsignal, and the end stage (GE), triggered by the driver stage (T), isconnected to the equipotential face (F).
 8. The switching power supplyof claim 4, wherein the inverter includes a transformer (TR1), theprimary winding of the transformer (TR1) being supplied with thesynchronization signal, and the secondary winding of the transformer(TR1) being connected to the equipotential face (F), whereby the primarywinding functions as the open-loop control circuit and the secondarywinding functions as the voltage generator.
 9. The switching powersupply of claim 8, wherein the decoupling circuit comprises a powertransformer (TR1), which also forms a transformer for forming theopen-loop control circuit and the voltage generator; primary side of thetransformer is connected to the switching portion and its secondary sideis connected to at least one incandescent lamp (HG);the secondary sideof the power transformer has an additional winding (ZW), whose first endis connected to the equipotential face (F) and whose second end isconnected to a point (FP) of the switching power supply, and the point(FP) has a potential such that a compensation current that iscapacitively coupled to the environment and is inverted relative to thecommon-mode noise flows across the equipotential face (F).
 10. Theswitching power supply of claim 1, wherein the interference source andthe additional source are connected serially, the switching power supplyhas as many additional sources (WE2; WE3) as there are supply leads thatcome from the alternating voltage mains network (N) or from a directvoltage source and are connected to the switching power supply,andwherein, in each supply lead, there is one additional source (WE2;WE3) serially connected and, optionally, a mains rectifier (GR) isprovided, selectively connected preceding, or following, the additionalsources (WE2; WE3).
 11. The switching power supply of claim 10,comprising a feed transformer (ES);wherein the additional sources areformed by the secondary windings (WE2, WE3) of the feed transformer(ES), simultaneously functioning as an inverter, whose primary winding(WE1) is supplied with the synchronization signal.
 12. The switchingpower supply of claim 11, wherein a coupling impedance (Z) is provided,andthe synchronization signal is supplied to the primary winding (WE1)of the feed transformer (ES) via the coupling impedance (Z).
 13. Theswitching power supply of claim 1, wherein the radio interferencesuppression circuit additionally includes a sensor transformer (SE);theclosed-loop control circuit includes a closed-loop control amplifier (T;T, WE1), wherein the windings (WS1, WS2) of the primary side of thesensor transformer (SE) are connected into the supply leads of theswitching power supply that come from the alternating voltage mainsnetwork (N) or a direct voltage source, and wherein, optionally, thewindings (WS1, WS2) of the primary side of the sensor transformer (SE)are selectively preceded or followed by a mains rectifier (GR) and thesecondary winding (WS3) of the sensor transformer (SE) is connected tothe closed-loop control amplifier (T; T, WE1), which in turn triggersthe additional source or sources (GE, F; WE2; WE3), whereby the radiointerference suppression circuit, together with capacitive couplings tothe environment via parasitic capacitances, forms said closed-loopcontrol circuit.
 14. The switching power supply of claim 1, furtherincluding a mains rectifier (GR).
 15. The switching power supply ofclaim 10, further including a main rectifier (GR) forming said directvoltage source.
 16. The switching power supply of claim 13, furtherincluding a main rectifier (GR) forming said direct voltage source.